Differential analog circuits have many advantages over single-ended designs that make them desirable to include in electronic systems. In particular, differential circuits have larger dynamic range, better common-mode and power supply rejection, and suppression of even-ordered distortion products. Most analog and radio frequency (RF) circuitry signals are single-ended at the board level, because of the difficulty in matching components and the requirement of doubling the number of components to implement a differential circuit. Therefore, analog and RF applications typically perform a single-ended to differential conversion at the input of the system's integrated circuits.
In low-level RF applications, such as cellular phone, two-way radios, or satellite radio receivers, the signals presented at the input of the radio receiver are very low-level signals, requiring that the single-ended to differential conversion be performed with high linearity and little noise added. Many conventional mixers are designed to receive a differential input, because differential signals aid in decoupling the system from noise in the integrated circuit substrate, thereby lowering the system noise figure, and making the circuit more immune to noise caused by other components located on the same substrate. Because the mixer is designed to receive a differential signal input, while the antenna generates a single received signal, the system must transform the single-ended signal into a differential signal somewhere between the antenna and the mixer.
For example, many conventional low-noise amplifier (LNA) designs, such as, for example, in monolithic transceiver designs, are differential LNAs that require an external balun or an equivalent transformer to convert the single-ended signal into a differential signal. A balun is a device that is used to convert an unbalanced signal to a balanced one, or vice versa. However, the use of a balun can introduce approximately 0.5-1.0 dB of loss into the system. A differential LNA can also exhibit more noise than a single-ended LNA, if the power consumption is the same.
Alternatively, a single-ended LNA can supply a single-ended signal to the mixer. FIG. 1 is a circuit diagram illustrating a mixer 100 that can be used in radio frequency communication systems. Mixer 100 includes a mixer section 101, comprised of transistors Q1, Q2, Q3 and Q4, and a radio frequency (RF) input section 102, comprised of transistors Q5 and Q6. Transistors Q5 and Q6 are configured as a differential amplifier. VRF is the single-ended voltage signal supplied from the LNA to one side of the RF input section 102, while the other side of the RF input section 102 can be AC grounded. RF input section 102 converts the single-ended voltage signal VRF into two output currents, IOUT1 and IOUT2. The mixer section 101 is a Gilbert-cell type double-balanced switching mixer. The mixer section 101 mixes the two output currents with a supplied local oscillator signal VLO to generate the differential mixed output signal at terminals 125.
The RF input section 102 includes first resistor 105 and second resistor 110 connected to the emitters of transistors Q5 and Q6, respectively. First and second resistors 105 and 110 are degeneration resistors that serve to reduce the gain and improve the input linearity of the differential amplifier of the RF input section 102. Increasing the size of first and second resistors 105 and 110 can improve linearity, but at the cost of gain. The tail ends of first and second resistors 105 and 110 can be connected to ground, but to improve linearity, the tail ends of first and second resistors 105 and 110 can be connected to an active current source 120. However, the inclusion of the active current source 120 requires that significantly more voltage “head room” be available for operation of the mixer 100. Additionally, active current source 120 can introduce a large junction capacitance from the transistor of the active current source 120. The capacitance can degrade the single-ended-to-differential conversion, because a portion of the input signal will be shunted away by the capacitance of the transistor of the active current source 120.